To an increasing extent in recent years, tractors have become larger and larger. One tractor in common use today employs two sections articulated with respect to each other and an engine having over 250 drawbar horsepower. The reason for these very large tractors is that the farms on which they are used have greatly increased in size without any corresponding increase in the number of people doing the farming. It has accordingly become desirable to employ a very large tractor so that very wide implements can be pulled by it. This enables the ground to be covered in a shorter period of time by a single operator.
From a practical standpoint, there tends to be an upper limit on the size of the engine which may be used in a tractor. Consequently, as more and more power is necessary with ever larger tractors, some means must be developed for providing for a plurality of engines in connection with any one tractor.
One arrangement which has been proposed is that of having two engine driven units behind a lead tractor, each of said engine driven units having its own ground engaging member. These units may be rigidly secured together side by side or as in the co-pending application of Paul A. Nystuen for "Articulated Tractor With Multiple Power Plants" filed of even date herewith, the engine driven units may be in the form of two tractor vehicles coupled together to the lead vehicle through a linkage arrangement.
The problem that arises whenever two or more tractor units are connected side by side, each tractor unit driving its own set of wheels or other ground engaging members, is that upon a turn, the two sets of wheels or ground engaging members tend to be rotated at the same speed. As is well known, it is necessary in a turn for the wheels on the outer side of the turn to rotate at a higher speed than those on the inside of the turn. Where a single engine or power plant is employed for driving wheels on the inside and outside of the turn, a differential is normally employed to permit a difference in speed of the two sets of wheels. This, however, is not practical where the inner wheels are driven by a separate power plant than the outer wheels.
One way that has been proposed of overcoming this problem discussed above is that of providing some means to enable the engine driving the wheels on the inside of the turn to be operated at a lower speed than that driving the wheels on the outside of the turn. These prior art arrangements, however, have not been particularly suitable for use in connection with tractors in which there are at least three independent internal combustion engines, each associated with one of the vehicles forming the articulated tractor. Furthermore, these prior art arrangements have not been particularly satisfactory in connection with an arrangement in which the main control of all of the tractor vehicles is normally from a central point in the lead tractor.